Rheological effect of different deflocculation mechanisms on a

Ester Barrachina, Jordi Llop, Maria-Dolores Notari, Diego Fraga, Rafael Martí, Ivan Calvet, Aitor Rey,
Teodora S.Journal
Lyubenova,
StephanTechnology
V. Kozhukharov,
Vladimir S.50,
Kozhukharov,
Juan B. Carda
of Chemical
and Metallurgy,
4, 2015, 493-502
RHEOLOGICAL EFFECT OF DIFFERENT DEFLOCCULATION MECHANISMS
ON A PORCELAIN CERAMIC COMPOSITION
Ester Barrachina1, Jordi Llop2, Maria-Dolores Notari2, Diego Fraga1,
Rafael Martí1, Ivan Calvet1, Aitor Rey1, Teodora S. Lyubenova1, Stephan V. Kozhukharov3,
Vladimir S. Kozhukharov3, Juan B. Carda1
Department of Inorganic and Organic Chemistry,
Universitat Jaume I, Castellón de la Plana, España
(Spain) Vicent Sos Baynat Blv.,
12071 Castelló de la Plana, Spain
E-mail: [email protected]
2
Escola Superior Ceràmica de L´Alcora,
Castellón, (Spain) Av. Corts Valencianes,
23, 12110 L’Alcora, Castellón, Spain
3
Laboratory of Advanced Materials Research (LAMAR)
University of Chemical Technology and Metallurgy
8 Kl. Ohridski, 1756 Sofia, Bulgaria
1
Received 10 November 2014
Accepted 4 May 2015
ABSTRACT
The current environmental policy imposes general optimisation of the industrial process. There are two critical steps
related to suspensions fluidity in the ceramic spray-drying industry: the emptying of the ball mill and the spray-drying
operation. The chemicals producers provide a large number of commercial deflocculants aiming to improve both processes.
The present research is focused on the comparison of the rheological effect of four commercial liquid deflocculants on a
porcelain ceramic composition in view of different mechanisms of deflocculation. The suspensions prepared have 70 %-solid
content and a range of deflocculant concentrations between 0.1 mass % and 0.6 mass %. Each suspension is characterized
by measuring its solid content, particle size distribution, pH values, conductivity and rheology, including viscosity at two
shear rates and thixotropy calculated from the hysteresis area of the flowability curves. On the basis of the experiments
carried out adequate conclusions are presented.
Keywords: spray-drying, rheological properties, deflocculants, viscosity, thixotropy, suspensions, porcelain industry.
INTRODUCTION
At present, the environmental policy forces the
industrial companies to improve their anti-pollution
systems directly through optimization of the production process [1 - 4]. In this context, the spray-drying
ceramic factories tend to use high solid content suspensions in order to reduce the evaporated water volume,
increase their production efficiency and reduce energy
consumption. As it is shown in Figs. 1 and 2, on the
basis of real average data referring to the production
rate, energetic consumption and energy savings levels
for a large number of spray-drying Spanish industries
we can assume that the solid content increase starting at
68 % will result in energetic savings in the range from
30 % to 50 % [5, 6]. The main way to stand behind the
fluidity of ceramic suspensions in industries is to add
and apply proper deflocculants and dispersants [7 - 10].
It is well known that the dispersion of ceramic
particles is a fundamental step in the industrial ceramic
processes aiming to obtain a homogeneous and stable
system of individual non-aggregated particles [11]. In
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Journal of Chemical Technology and Metallurgy, 50, 4, 2015
Fig. 1. Correlation between the production rate, expressed
in kg sprayed powder per kg evaporated water and energy
consumption (expressed in kWh expensed energy for
each kg dried powder product for spray-drying process
performed in typical industrial regime .
Fig. 2. Energy consumption reduction for Spray-drying
operation performed in optimised industrial conditions,
due to the increased solid content of the suspensions used.
the ceramic industry, chemical additives are usually
used to reduce the suspension viscosity, while keeping
the solid content as high as possible [12]. Mixtures of
sodium tripolyphosphate and metasilicate in a solid state
are traditional and widely used materials for suspensions
of ceramic paste deflocculation. Recently, this trend has
changed in the ceramic sector through mixing liquid
organic compounds (mainly those on acrylic acidic
polymers base) with traditional materials thus reducing
the process costs. However, the use of solid compounds
in industries has been limited owing to the health risk
involved in their handling [13].
Currently, inorganic substances (silicates, phosphates and carbonates sodium salts) or organic additives,
generally polyacrylates salts of different molecular
structures are commonly used in a liquid state. In the
ceramic technology they are added from 0.1 to 0.6 mass
494
percents in order to decrease the dynamic viscosity down
to values ca. 300 ± 100 mPa s [14].
The primary reason to use sodium silicate is to add
silicate but not sodium ions. Nevertheless, since sodium
silicate is a soluble compound, its application provides
an excellent way to add silicate ions to suspensions.
In fact they are generally added to remove unwanted
flocculating cations such as magnesium and calcium
cations. Consequently, the effect of sodium silicate refers
to pH variation of the suspension used and a specific
adsorption of negatively charged deflocculant anions
on the clay positive edges being the driving force of
electrostatic nature.
Organic deflocculants like polyacrylates and phosphonates have to be added if the flocculating cations
amount in the suspensions is not adequate. They interact
with the coating flocculating cations and particle surfaces
and hence increase the effective electrostatic charges. The
polyacrylates are a chemical class of acrylate polymers
obtained through polymerization of acrylic acid esters
and salts, whereas the phosphonates are organic compounds containing C-PO(OH)2 or C-PO(OR)2 groups,
where R stands for alkyl or aryl units [15], respectively.
Nowadays, three basic mechanisms are known and
can act either individually, or in combination, in dependence of the deflocculant used [16, 17]. They are:
• Electrostatic action: cation exchange affecting
the thickness of the electrical double layer of the raw
materials particles;
• Steric hindrance: steric repulsion by the introduction of functional groups which act as spacers among
the raw material particles;
• Cation capture: binding of interfering cations by
complexing.
The rheology describes the deformation of a body
under stress impact where bodies in this context mean
solids, liquids or gases. The rheological behaviour of
suspensions is focused on viscosity which can be described as the physical property of a liquid to resist a
shear-induced flow. It may be influenced by the following independent factors: (i) physico-chemical nature of
the investigated substance; (ii) temperature variations;
(iii) pressure in the compressed fluids; (iv) shear rate; (v)
time and history of the dispersion and (vi) electrical field
applied. Suspensions for which the latter phenomenon
is typical are those whose flow behaviour is strongly
influenced by the magnitude of the electric fields acting
Ester Barrachina, Jordi Llop, Maria-Dolores Notari, Diego Fraga, Rafael Martí, Ivan Calvet, Aitor Rey,
Teodora S. Lyubenova, Stephan V. Kozhukharov, Vladimir S. Kozhukharov, Juan B. Carda
Fig. 3. Common flow behaviour of Newtonian (1) and Non-Newtonian liquids (2, 3, and
4) according to [21] 1 – Newtonian liquid; 2 – Pseudoplastic liquid, 3 – Dilatant liquid,
4 – Plastic liquid.
upon them [18 - 20].
The more common rheological behaviours are
schematically illustrated in Fig. 3 together with the corresponding flow and viscosity curves.
Each liquid which is presented by a straight line of
a slope (α) starting at the origin of the flow curve and
whose viscosity is independent of the shear rate (γ) can
be considered a “Newtonian liquid” (Fig.3, curve 1).
The case where the apparent viscosity ηapp of the pseudoplastic liquid (Fig.3, curve 2) decreases drastically
with shear rate increase from low to high levels refers
to a non-Newtonian liquid. Liquids whose viscosity
increases with shear rate increase under certain conditions of stress (Fig. 3, curve 3) show dilatant behaviour,
i.e. these are non-Newtonian liquids as well. It can be
assumed that the difference between the pseudoplastic
liquids and the typical plastic ones is that the former
possess an additional yield point (Fig.3, curve 4) being
also non-Newtonian liquids. [11,19, 21 - 23].
The aim of the present research is to compare the
rheological effect of four commercial liquid deflocculants exhibiting preferential action mechanisms [24 - 31]
on a porcelain ceramic standard composition in view of
the different mechanisms of deflocculation.
EXPERIMENTAL
Commercial Deflocculants
Four commercial deflocculants of different particle
dispersion mechanism are selected and their basic properties are specified in Table 1.
The deflocculants investigated are marked by A, B,
C and D aiming clarity. Compositions A, B and C contain
sodium silicate with polymers or phosphonates, whereas
D contains only phosphonates. Besides, compositions A
and B act according to the mechanism of electrostatic
action (due to the silicate anions) and steric hindrance
(rendered by the polymeric moieties), whereas C deflocculant action is based on the electrical charges of the
silicate anions combined with the complexant effect
of the phosphate groups. D deflocculant stabilizes the
suspensions by complexant caption ability. Besides, supplemental beneficial effect is also expected for phosphate
containing compositions C and D since the phosphate
Table 1. Basic description of the tested deflocculants marked with indexes A, B, C and D.
Deflocculant
Composition
Mechanism
A
Silicate (min. 90 %mass)
with polymer
Silicate (min. 90 %mass)
with polymer
Silicate (min. 90 %mass)
with phosphonates
Phosphonates
Electrostatic Action / Steric
Hindrance
Electrostatic Action / Steric
Hindrance
Electrostatic Action / Complexant
Effect
Complexant Effect
B
C
D
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Journal of Chemical Technology and Metallurgy, 50, 4, 2015
Table 2. Control rate mode programme.
Cycle subsequence
First cycle
Stop
Second cycle
compounds are known to have corrosion protective
capability [32, 33]. The latter provides active corrosion
inhibition by passivation of the steel ball milling and
spray-drying industrial equipment (such as pipelines
and other facilities). All deflocculants investigated in the
present research are supplied by Zschimmer & Schwarz
international Company [17]. The analysis of the derived
deflocculant-containing suspensions was performed by
systematic pH and electrical conductivity measurements.
Suspensions preparation
The suspensions prepared consisted of a standard
porcelain spray-dried powder mixed with water (a solid
content of 70 mass %) and different types of deflocculant
additives (A, B, C or D) of amounts ranging from 0.1
mass % to 0.6 mass %. They were milled for 7 minutes
in a planetary alumina ball mill to achieve particles size
less 63 µm. This industrial milling requirement refers
to all particles in case of porcelain pastes preparation.
Electrolytical and rheological measurements of
the suspensions obtained
All tested parameters were measured immediately
after the preparation of each paste in order to avoid any
ageing impact on the suspensions prior to the respective
investigation. In each case the solid phase fraction was
quantified from the difference between the wet and dried
weight of a sample slip. The particle size distribution was
measured by a LS 13 320 Laser Diffraction Particle size
analyzer of Beckman Coulter. These measurements were
performed in order to control the variation of rheological
properties driven by the size distribution modification.
The latter is attributed to the dependence of the suspensions fluidity on the deflocculation rate.
The electrolytical parameters were tested by a
Eutech pH6+ and a Eutech Cond7, product of Thermo
Fisher Scientific Company. The rheological variables of
496
Shear Rate
(s-1)
0-1000
0-0
1000-0
0
0-1000
0-0
1000-0
Time
(s)
120
1
120
120
120
1
120
each suspension were obtained using a traditional control
rate mode (i.e. RC-mode) comprising seven subsequent
stages. Haake Viscotester 550 was used. The results
obtained are summarised in Table 2.
The first rheometrical cycle was used to adjust
the testing time of all suspensions. The viscosity and
thixotropy values were derived on the ground of the
second cycle. The apparent dynamic viscosity was
studied at two different shear rates: a low one of100 s-1
and a high one of 1000 s-1, respectively. The lower shear
rate simulates the emptying of the industrial ball mills,
while the higher one resembles the rate used during the
spray-drying operation. The magnitude of thixotropy
was determined by MCR- rheometer (Haake Viscotester
550). Its values provided the determination of the hysteresis area enclosed between the flowability curves
(shear stress versus shear rate) of the second cycle [21].
Besides, two typical theoretical models were used for
curve adjustment analysis, namely: the Bingham plastic
and the Herschel-Bulkley model [14,18]
RESULTS AND DISCUSSION
Analysis of the commercial deflocculants
The conductivity of the deflocculants investigated
is in the range from 35 mS cm-1 to 50 mS cm-1, being
the deflocculant with complexant effect (index D), the
one that had the highest pH value as shown in Fig. 4.
All sodium silicate containing deflocculants (A, B and
C) reveal strong alkalinity, have pH values in the range
above 12, while the purely phosphonated one (D) shows
a slightly lower 10.5 pH value.
Analysis of ceramic suspensions
According to the results obtained the average value
of the solid content is equal to 70.08 ± 0.15 mass %.
This value corresponds to the constraints imposed by the
Ester Barrachina, Jordi Llop, Maria-Dolores Notari, Diego Fraga, Rafael Martí, Ivan Calvet, Aitor Rey,
Teodora S. Lyubenova, Stephan V. Kozhukharov, Vladimir S. Kozhukharov, Juan B. Carda
Fig. 4. Electrolytical properties of the used deflocculants.
Fig. 6. Electrolytical properties of the investigated deflocculant containing slips.
Fig. 5. Correspondence between the deflocculant content and
particle size distribution mean values of the resulting suspensions.
Fig. 7. Dependence between the deflocculant addition and
the derivative suspension pH value.
high industrial requirements. Furthermore, the standard
deviation is narrow enough providing the comparison
of the properties of the different suspensions prepared.
In general, the particle size distribution remains
unchanged with the addition of different quantities of
deflocculant to the suspensions. Unique deviations are
observed in the range of deflocculant content from 0.1
mass % to 0.2 mass % as demonstrated in Fig. 5. At this
deflocculant concentration the suspensions prepared by
adding A and D shows a rather distinguishable particle
size due probably to agglomeration phenomena. In
comparison, the compositions with addition of B and C
are much more stable during the milling process. The
average value of particle size obtained is ca. 15.6 ± 0.2
μm as illustrated in Fig. 5. Furthermore, a supplemental
insignificant effect of decrease of the average particle
size is observed at 0.2 mass % content of each deflocculant used. Regarding the electrolytical parameters,
the electrical conductivity obtained in each suspension
is directly proportional to the percentage of the deflocculant content and the values range is from 5.1 mS/cm to
6.1 mS/cm as expected with the addition of electrolytes
(Fig. 6).
Fig. 7 shows the correlation dependence between the
defloculant content and the pH values of the suspensions
obtained. It is found that the suspension pH increases
proportionally from 8.5 to 10.25 for suspensions A, B
and C. Unlike them D deflocculant’s content does not
affect the suspension pH value which stays equal to 8.5.
This fact is a consequence of the presence of complexing agents of functional groups in composition D. These
moieties have free valences in their electronic shells,
able to form electron pair bonds with free multivalent
cations without modifying either [H+], or [OH-] [17].
The tests carried out at low shear rate (100 s-1) related to the fluidity during the emptying of the ball mills
are illustrated in Fig. 8. As expected, prior to reaching
the deflocculant content excess, the apparent dynamic
viscosity is inversely proportional to the addition of defloculants A and B. It is worth adding that they have the
highest viscosity at 0.1 mass %. The optimal rheological
behaviour at low shear rate is reached at 0.4 mass % of
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Journal of Chemical Technology and Metallurgy, 50, 4, 2015
Fig. 8. Correlation between the deflocculant content and
the derivative suspension viscosity, determined at low
shear rate - 100 s-1.
Fig. 10. Correlation between the deflocculant content and
the resulting thixotropy of the investigated suspensions.
Fig. 9. Correlation between the deflocculant content and
the derivative suspension viscosity, determined at high
shear rate - 1000 s-1.
Fig. 11. Flow curves at 0.4 mass % deflocculation content
with Bingham plastic model.
D deflocculant. The apparent dynamic viscosity shows
a tendency to decrease at deflocculant contents below
0.2 %. It achieves values lower than 300 mPa.s at 0.6 %
content in the case of composition D (Fig. 8).
Fig. 9 illustrates the viscosity behaviour at a high
shear rate (1000 s-1). It is evident that all four commercial
liquid deflocculants have similar apparent dynamic viscosity values, between 300 mPa.s and 400 mPa.s. Only
composition D tends to decrease the suspension apparent
dynamic viscosity below 300 mPa.s after reaching 0.4
wt % content. Referring to the thixotropy behaviour,
deflocculants A, B and C show a narrower rheological
stability region (between 0.2 mass % and 0.4 mass %) in
comparison to that of deflocculant D. The latter shows a
decrease of this value because of the higher deflocculant
content incorporated in the suspension (Fig. 10).
Fig. 11 shows the flow curves of suspensions loaded
with 0.4 mass % of deflocculant adjusted to the Binghman plastic model. Suspensions with deflocculants B and
D show the smallest slopes in the tested range of shear
rate (<100 s-1) although the lowest yield point indicated
in Table 3 refers to deflocculant D.
On the other hand, a very clear relationship is outlined
if the Herschel-Bulkley model is applied to these flow
curves. This is demonstrated in Fig. 12. Table 4 summarises the data obtained for Herschel-Bulkley model
regression line of the tested deflocculants.
498
Table 3. Bingham plastic regression line of tested deflocculants.
Deflocculant
Yield point
Slope
Correlation
index
τ0 (MPa)
µp (MPa·s)
A
11.580
560,5
0,9996
B
10.070
305,6
0,9992
C
12.570
391,7
0,9996
D
6.897
336,4
0,9987
factor
Ester Barrachina, Jordi Llop, Maria-Dolores Notari, Diego Fraga, Rafael Martí, Ivan Calvet, Aitor Rey,
Teodora S. Lyubenova, Stephan V. Kozhukharov, Vladimir S. Kozhukharov, Juan B. Carda
Table 4. Herschel-Bulkley model regression line of tested deflocculants.
Deviation from a
Deflocculant
Yield point
Fluid consistence
index
τ0 (MPa)
K (MPa·sn)
A
10.430
731,7
0,9447
0,9998
B
9.250
430,1
0,9292
0,9995
C
12.990
337,7
1,0310
0,9997
D
5.286
600,3
0,8805
0,9995
Newtonian fluid
n
Correlation
factor
the viscosity curves recorded with a low shear rate are
considered. Slips with deflocculants A and B show an
intermediate behaviour. The sample indexed D has the
lowest dynamic viscosity value as shown in Fig. 13.
CONCLUSIONS
Fig. 12. Flow curves at 0.4 mass % deflocculation content
with Herschel-Bulkley model.
Fig. 13. Viscosity curves obtained from the investigated
suspensions with 0.4 mass % deflocculation content.
If criterion “n” of deviation from a Newtonian
fluid approaches a unit, then the Herschel-Bulkley flow
equation transforms into Bingham plastic model. In
fact this is the case as seen from Table 4. Hence, it can
be concluded that the plastic model is applicable to the
suspensions tested.
The suspension indexed C shows the highest dynamic viscosity followed by that with index A when
The present research is devoted to the comparison of
the rheological effects on a standard porcelain ceramic
composition provided by different mechanism of deflocculation. Four commercial liquid deflocculants of suitable capabilities for industrial applications were used.
It is found that the defloculants indexed A, B and C
show identical behaviour referring to the tests performed
aiming to determine the particle size distribution, the
electrical conductivity, the pH values, the apparent
dynamic viscosity at a low and a high share rate. The
same is valid for the thixotropy behaviour observed. The
results obtained are expected since the basic building
ingredient of the deflocculants is a silicate with (A,B)
and without (C) a polymer base.
Deflocculant indexed D having a complexant action
shows the lowest pH value, the highest conductivity
and clear deviations in regard to the other tests results
obtained. Consequently, on the basis of the acidic nature of the phosphonates, structuring incorporates, and
complexant effect proposed, it can efficiently inhibit any
corrosion of pipes and containers in the respective industrial facilities. Additional investigations are required
in respect to the corrosion behaviour of the suspensions
used as some phosphates are good corrosion inhibitors.
It is found that the deflocculant’s type does not affect
the particles size distribution of the suspensions. This
is valid for all concentrations with the exception of 0.1
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Journal of Chemical Technology and Metallurgy, 50, 4, 2015
mass % . It is considered that the quantity in the latter
case is not enough to mill properly the paste.
Deflocculant D exhibits a superior performance if an
optimum viscosity is required during the industrial mill
process, i.e. in the case of a low shear rate. At high shear
rates during the ceramic spray-drying process deflocculant B providing charges and steric effects shows the
lowest viscosity at ca. 0.2 mass % - 0.4 mass % content,
whereas deflocculant D tends to decrease viscosity at
0.4 mass % and higher concentrations. It is shown that
deflocculant D has lowest thixotropy. The other deflocculants exhibit a narrower range of rheological stability
which may hamper the fluidity of industrial suspensions
due to ageing processes.
All ceramic suspensions prepared have a standard
porcelain composition and a plastic liquid behaviour as
they show a yield point and their viscosity decreases at
shear rate increase. Deflocculant D provides the optimal industrial working conditions because its addition
leads to the lowest yield point among the compositions
investigated.
The experiments carried out provide to conclude
that all four deflocculants tested can be used to stabilize
ceramic porcelain suspensions of a standard porcelain
composition. The deflocculants indexed as A, B and
C act mainly electrostatically determining thus the
corresponding deflocculation mechanism, but sodium
silicate only is not enough to reach the optimum rheological conditions. Hence, a huge content of sodium
silicate (90 mass % or more) has to be mixed with
polymers or phosphonates to improve the rheology
of the ceramic suspensions. Thus results very close
to those obtained by the complexant mechanism can
be obtained. The latter mechanism is more suitable to
reduce viscosity and thixotropy as well as to preserve
the industrial facilities.
Acknowledgements
The chemical company Zschimmer & Schwarz
España S.A. is highly appreciated for the technological advices and supply of the deflocculants tested in
the present research. The authors are grateful to the
Spanish Government for the financial support of this
work through the RETO INVESTIGACIÓN (ENE201349136-C4-2-R).
500
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